Food resources of shrimp-eating cichlid fishes in Lake Tanganyika

Species, size composition, and diurnal variation in abundance of shrimp (Atyidae) were examined and compared with food habits of 14 species of shrimp-eating cichlids in Lake Tanganyika. In the daytime when the cichlids foraged actively, only one species of shrimp, Limnocaridina latipes, was present on rock surfaces. Eleven of the 14 species of shrimp-eaters examined preyed mainly on L. latipes; the shrimp-eaters foraging only on rock surfaces preyed exclusively on L. latipes, whereas shrimp-eaters also taking foods from the interstices between or under stones and debris preyed on various species of shrimp. Although three kinds of shrimp-eaters, each with a unique prey catching technique and preferred shrimp size, were recognized, all shrimp-eaters preyed on shrimp of greatly overlapping sizes. This fact confirms that partitioning of food resources takes place via different capturing techniques. Moreover, the utilization of the same food resource in a different manner may promote group foraging in some shrimp-eaters, because the shrimp is a suitable animal to hunt in groups and is a quickly renewable food of potential abundance.     

Resource availability and population dynamics of Nicrophorus investigator, an obligate carrion breeder

1. Food resources for rearing young may influence insect populations. This is particularly true for insects that breed obligately on rare, ephemeral resources such as dung, fungi, or carrion. 2. Beetles in the genus Nicrophorus bury small vertebrate carcasses for rearing their young. Studies reviewed by Scott (1998) have found a positive relationship between carcass mass and total brood size. It is likely that access to carcasses suitable for breeding, and not food or mates, limits reproduction in both male and female Nicrophorus. Thus, small mammal densities could determine Nicrophorus population sizes. 3. The work reported here examined the relationship between Nicrophorus investigator (Coleoptera: Silphidae) population size and small mammal abundance at two sites over a 4‐year period. 4. Nicrophorus investigator buried and reared young on all the native small rodent species trapped at two sites in south‐western Colorado, U.S.A. (Peromyscus maniculatus, Microtus montanus, Zapus princeps, Tamias minimus, Thomomys talpoides). They preferred to bury and reproduce on rodent carcasses weighing between 16 and 48 g; rodents of this size represented 82% of captures. 5. Population sizes of N. investigator and small rodents were estimated simultaneously using mark‐recapture censuses over a 4‐year period. Considering only rodents within the size range used by N. investigator, the estimated small mammal biomass per hectare in one year and the beetle population size in the following year were correlated significantly.     

Habitat preferences of the urban wall spider Oecobius navus (Araneae, Oecobiidae)

Abstract  To assess the relative impact of a range of habitat variables on spider abundance, field and laboratory experiments were conducted on populations of the urban wall spider Oecobius navus in suburban Perth, Western Australia. Habitat characteristics investigated were: substrate type, wind speed, rainfall, sunlight exposure, relative humidity, air temperature, substrate temperature, artificial lighting and prey type/abundance. In the field, O. navus was found to be associated with high humidity, low air temperature and shelter from sunlight and rainfall. Oecobius navus was more abundant at sites with greater prey abundance. The most common prey item was the red meat ant Iridomyrmex chasei . Juvenile spiders were more abundant than adult spiders; however, patterns between spider abundance and habitat variables were similar for both adults and juveniles. Laboratory experiments showed that O. navus preferred to build webs on wooden substrates, and pitted limestone walls. These findings indicate that O. navus may be vulnerable to desiccation and/or thermal stress, and thus survives better on sheltered walls.     

Validation of a randomization procedure to assess animal habitat preferences: microhabitat use of tiger sharks in a seagrass ecosystem

1. Tiger sharks Galeocerdo cuvier are important predators in a variety of nearshore communities, including the seagrass ecosystem of Shark Bay, Western Australia. Because tiger sharks are known to influence spatial distributions of multiple prey species, it is important to understand how they use habitats at a variety of spatial scales. We used a combination of catch rates and acoustic tracking to determine tiger shark microhabitat use in Shark Bay. 2. Comparing habitat-use data from tracking against the null hypothesis of no habitat preference is hindered in Shark Bay, as elsewhere, by the difficulty of defining expected habitat use given random movement. We used randomization procedures to generate expected habitat use in the absence of habitat preference and expected habitat use differences among groups (e.g. males and females). We tested the performance of these protocols using simulated data sets with known habitat preferences. 3. The technique correctly classified sets of simulated tracks as displaying a preference or not and was a conservative test for differences in habitat preferences between subgroups of tracks (e.g. males vs. females). 4. Sharks preferred shallow habitats over deep ones, and preferred shallow edge microhabitats over shallow interior ones. The use of shallow edges likely increases encounter rates with potential prey and may have profound consequences for the dynamics of Shark Bay's seagrass ecosystem through indirect effects transmitted by grazers that are common prey of tiger sharks. 5. Females showed a greater tendency to use shallow edge microhabitats than did males; this pattern was not detected by traditional analysis techniques. 6. The randomization procedures presented here are applicable to many field studies that use tracking by allowing researchers both to determine overall habitat preferences and to identify differences in habitat use between groups within their sample.     

Trends in the prey size-based trophic niches of feral and House Cats Felis catus L.

House Cats Felis catus L., whether attached to human households or not, appear to be versatile opportunistic predators. Their principal prey in most areas are mammals (rodents and rabbits), with bird prey secondary. Trophic niche breadth, as measured by the standard deviation of the spectrum of logarithmically transformed prey sizes ('SLH'), shows a latitudinal trend, being greater in low latitudes: it is also greater in periods of high prey availability. This appears to be influenced by inclusion of very small prey, especially insects, in areas and seasons when they are available. Both the niche breadth and the mean prey size (niche position) appear to be constant as population mean cat size increases. The most common prey size for cats is about 1% of their own body weights, which is much less than most previously reported values for carnivores.     

Contrasting effects of habitat fragmentation,population density,and prey availability on body condition of two orb‐weaving spiders

1. Habitat fragmentation is a major threat to biodiversity because it disrupts movement between habitat patches. In addition, arthropod fitness may be reduced in fragmented habitats, e.g. due to reduced prey availability. 2. We studied the relationship of spider body condition with habitat fragmentation, population density, and prey availability. We expected that prey availability and population density of spiders would be affected by landscape composition and patch isolation. Body condition should be enhanced by high prey availability, but negatively affected by population density due to competition. 3. We sampled spiders on 30 groups of cherry trees that varied independently in the level of isolation from other woody habitats and in the percentage of woody habitat within 500 m radius. As a measure of body condition, we used residuals of the relationship between individual body mass/opisthosoma width and prosoma width of the two most common orb‐weaving spider species, Nuctenea umbratica Clerck and Araniella opisthographa Kulczynski. 4. Body condition of A. opisthographa was positively correlated with the abundance of flies, which increased with the percentage of forest in the landscape. In contrast, body condition of N. umbratica was reduced at high population densities, presumably due to intraspecific competition. In addition, body condition and population density of A. opisthographa was lower at isolated sites. 5. Our study suggests that effects of landscape fragmentation on body condition vary strongly between spider species, depending on the relative role of food limitation and intraspecific competition.     

Open ground individuals and population structure in Drosophila subobscura Collin

The distribution of Drosophila subobscura adults within, and their movement through, open ground habitats has been investigated, along with the status of such individuals within populations as a whole. Movement occurs between open ground and woodland, to some extent as a result of passive transport by air currents, but flies can remain in open ground for at least several days at a time. There are no detectable differences in either the reproductive age-structure or the body-size distributions of flies collected from woodland and open ground habitats, although a significantly higher proportion of females are found in open ground than in woodland. The large number of gravid females in open ground provide the potential for breeding there, but circumstantial evidence suggests that this is unlikely to occur. Rather, their presence appears to reflect the foraging strategy adopted by females in response to their high nutritional requirements. The flux of flies through open ground between woodlands suggests that previous estimates of the genetically-important parameter, effective population size, have been seriously underestimated for natural D. subobscura populations.     

Prey size spectra and prey availability of larval and small juvenile cod

The aim of the present study is to describe the prey preference characteristics of cod larvae and assess preference variability in relation to species and size composition of copepod prey. A further aim is to examine the hypothesis that dietary prey size spectra remain the same during the larval stage when viewed on a relative predator/prey size scale. The study is based on stomach analysis of larval/juvenile cod in the size range 10–35 mm from nursery grounds in the North Sea. Stomach contents (species, size) were compared to environmental composition and preference indices were calculated. Prey size spectra had the expected relationship to larval cod size, and preference for given copepod species could be ascribed to their relative size. Additional species-specific preferences were evident, for example the larger Pseudocalanus and the larger Calanus spp. were highly preferred. Available prey biomass was highest in the areas of a hydrographic front, where larvae have been shown to concentrate. Changes in prey availability with larval growth depend strongly on specific prey biomass spectra at a given location. Both increasing and decreasing prey availability at increasing larval size were indicated, dependent on location. The findings illustrate the usefulness of coupling dietary prey size spectra and biomass spectra of available prey sizes during studies of ichthyoplankton feeding ecology.     

Ecological and biogeographical studies on the tropical rain forest of south Yunnan,SW China with a special reference to its relation with rain forests of tropical Asia

Abstract. Ecological and biogeographic analyses of the tropical rain forest in south Yunnan were made using data from seventeen sample plots and floristic inventories of about 1000 species of seed plants. The rain forest is shown to be a type of true tropical rain forest because it has almost the same profile, physiognomic characteristics, species richness per unit area, numbers of individuals in each tree species and diameter classes of trees as classic lowland tropical rain forests. As the area is at the northern margin of monsoonal tropics, the rain forest differs from equatorial lowland rain forests in having some deciduous trees in the canopy layer, fewer megaphanaerophytes and epiphytes but more species of lianas as well as more species of microphylls. In its floristic composition, about 80% of total families. 94% of total genera and more than 90% of total species are tropical, of which about 38% of genera and 74% of species are tropical Asian. Furthermore, the rain forest has not only almost the same families and genera, but also the same families rank in the top ten both in species richness and in dominance of stems, as lowland forests in southeast Asia. It is indisputable that the flora of the rain forest is part of the tropical Asian flora. However, most of the tropical families and genera have their northern limits in south Yunnan and most have their centre of species diversity in Malesia. More strictly tropical families and genera have relatively lower species richness and importance compared with lowland rain forests in tropical southeast Asia. Thus, the flora also shows characteristics of being at the margin of the tropics. Based mainly on physiognomy and floristic composition the tropical rain forest of Yunnan is classified into two types, i.e. seasonal rain forest and wet seasonal rain forest, the latter is further divided into two subtypes, i.e. mixed rain forest and dipterocarp rain forest. From analysis of geographic elements it is also shown that the tropical rain forest of Yunnan occurs at a geographical nexus with its flora coming mainly from four sources, i.e. Malesia, south Himalayas, Indochina and China.     

Rolling stones and stable homes: social structure,habitat diversity and population genetics of the Hawaiian spinner dolphin (Stenella longirostris)

Spinner dolphins (Stenella longirostris) exhibit different social behaviours at two regions in the Hawaiian Archipelago: off the high volcanic islands in the SE archipelago they form dynamic groups with ever‐changing membership, but in the low carbonate atolls in the NW archipelago they form long‐term stable groups. To determine whether these environmental and social differences influence population genetic structure, we surveyed spinner dolphins throughout the Hawaiian Archipelago with mtDNA control region sequences and 10 microsatellite loci (n = 505). F‐statistics, Bayesian cluster analyses, and assignment tests revealed population genetic separations between most islands, with less genetic structuring among the NW atolls than among the SE high islands. The populations with the most stable social structure (Midway and Kure Atolls) have the highest gene flow between populations (mtDNA Φ_ST < 0.001, P = 0.357; microsatellite F_ST = ?0.001; P = 0.597), and a population with dynamic groups and fluid social structure (the Kona Coast of the island of Hawai’i) has the lowest gene flow (mtDNA 0.042 < Φ_ST < 0.236, P < 0.05; microsatellite 0.016 < F_ST < 0.040, P < 0.001). We suggest that gene flow, dispersal, and social structure are influenced by the availability of habitat and resources at each island. Genetic comparisons to a South Pacific location (n = 16) indicate that Hawaiian populations are genetically depauperate and isolated from other Pacific locations (mtDNA 0.216 < F_ST < 0.643, P < 0.001; microsatellite 0.058 < F_ST < 0.090, P < 0.001); this isolation may also influence social and genetic structure within Hawai’i. Our results illustrate that genetic and social structure are flexible traits that can vary between even closely‐related populations.